On rare occassion, I have seen Time Stop disappear with a message of "Time speeds up..." when casting a defensive spell. I have experienced it both in a room with a battle though myself disengaged and not even grouped to the PC I am casting upon, and also in a room without combat and upon yet another PC. Neither of those times were I assisting someone, tanking, nor having a riposte upon myself. I do not recall exactly what spells were being cast, other than them being from the following list:
Dragonscales
Globe
Blur
Haste
Detect Invisibility
Detect Magic
Energy Shield

To my recollection, I don't recall trying to Blink someone and having it fail, nor attempting to Invis someone. I must investigate these and other spells moreso to determine if the bug affects those also.

It's more critical in zoning when you are the only enchanter for the group and the leader is ....more hasty in getting the tanks in than in having the enchanter spell people up. But still, it's a frustration even when nonchalantly casting detect invis at the Waterdeep Inn.

If it's a disruption such as a spell failure, I'd be more understanding of it. However the message seems more akin to a bug than a feature of circumstance.

Time always speeds up when your group is in battle, even though you are only using defensive spells and are not engaged. It sometimes happens when you're not grouped as well, if the mob casts areas or uses area skills. I've never seen this happen when casting gates in the inn.

To clarify, Time Stop is designed to speed up the chant of spells when not in combat. If you are in a room with combat going on and you cast a spell while having Time Stop up, the spell will fall. This will happen regardless of who the target of the spell is.

If you cast a spell on yourself and there is combat in the room TS will break.
If you cast a spell on a group member with combat in the room TS will break.
If youc ast a spell on a PC not in your group with combat in the room TS will break.

Ohh, maybe you're saying that sometimes TS lasts longer than others? IF that's the question, then there is definitly a random amount of time that each one lasts, sometimes you get a crap one and it falls after 15s, sometimes you get a min or two.